In a signal processing technology, complex signals are generally divided by quadrature sampling into two channels of signals (that is, IQ signals, which may also be written as I/Q signals): I (In-Phase) signals and Q (Quadrature-Phase) signals. In a communication system, IQ signals are modulated into a quadrature carrier by using a frequency mixer at a transmit end and then transmitted, as shown in FIG. 1. At a receive end, IQ signals are demodulated and separated by using the quadrature carrier, as shown in FIG. 2. For correct decomposition and synthesis of complex signals, it is required that the two channels of signals, that is, the IQ signals, have a same amplitude gain and an absolute quadrature phase difference.
During the implementation of the present invention, the inventor has discovered at least the following problems in the prior art.
In an actual system, because a frequency mixer, a DAC (Digital-Analog Converter), an ADC (Analog-Digital Converter), and a wave filter are not ideal and transmission channel characteristics are inconsistent, IQ signals do not have a completely same amplitude gain. Because a phase shifter is not ideal and a channel phase is distorted non-linearly, phases of IQ signals are not completely quadrature; and IQ signals will have a direct-current offset due to factors such as local oscillator leakage in the system. Therefore, in an actual communication system, an I signal s′I and a Q signal s′Q obtained at a receive end are often distorted coupling of a transmit I signal sI and a transmit Q signal sQ. Formulas (1) and (2) show specific relations.s′I=DCI+aI×(sI cos(ψI)+sQ sin(ψI))  (1)s′Q=DCQ+aQ×(sQ cos(ψQ)+sI sin(ψQ))  (2)where DCI and DCQ respectively indicate a direct-current offset of an I signal and a direct-current offset of a Q signal; aI and aQ respectively indicate a gain amplitude of the I signal and a gain amplitude of the Q signal; and ψI and ψQ respectively indicate a phase offset of the I signal and a phase offset of the Q signal.
Distortion of IQ signals will directly cause distortion of a receive signal constellation diagram, greatly affecting performance of subsequent equalization and demodulation. Although currently a method of improving circuit component quality and circuit design can reduce a signal distortion degree, this manner will bring an extremely high hardware cost.